The invention relates to a retruder for continuously conveying, mixing and/or kneading media at elevated pressure. The term extruder has become established for apparatus of a similar type. This term is fundamentally incorrect for the present invention, because of the reversal of the direction of conveying, instead of a circumlocution the abbreviation retruder (reverse extruder) is proposed.
Continuous conveying, mixing and/or kneading of media is part of the basic operations of process engineering. The material for mixing and/or conveying can be very different substances. The chemical and physical properties of these substances, their state of aggregation and their behaviour under pressure and heat can be correspondingly different. The solutions provided by the engineering of the prior art, which extend from kneading chocolate composition, producing semifinished plastics by extrusion and processing flowable plastics granules with a glass fibre content for injection moulding to the disposal of biologically active waste, correspond to this variety.
For all these tasks there are known machines which have, depending on the requirements, cooled or heated housings and at least two rotating, mutually meshing conveying screws which produce a pressure build-up by forcibly conveying the medium in the individual, closed windings of the screw channel, without having a reactive effect on the mass transport. The seals and the bearings are the weak points of these constructions, since the conveying pressure produces high axial pressures. In particular, this disadvantage becomes important when processing abrasive media or when the high pressures in the medium for the purpose of increasing the frictional forces in the medium are themselves advantageous. This is the case with kneading tasks if the efficiency of kneading is increased or for example in the case of a frictional reactor which is to be operated as a thermo-mechanical process unit for preparing biomass, at high operating pressures.